The present invention relates to a variable valve actuating mechanism. More particularly, the present invention relates to a variable valve actuating mechanism having a torsional lash control spring.
Modern internal combustion engines may incorporate advanced throttle control systems, such as, for example, intake valve throttle control systems, to improve fuel economy and performance. Generally, intake valve throttle control systems control the flow of gas and air into and out of the engine cylinders by varying the timing, duration and/or lift (i.e., the valve lift profile) of the cylinder valves in response to engine operating parameters, such as engine load, speed, and driver input. Intake valve throttle control systems vary the valve lift profile through the use of variously-configured mechanical and/or electromechanical devices, collectively referred to herein as variable valve actuation (VVA) mechanisms. Several examples of particular embodiments of VVA mechanisms are detailed in commonly assigned U.S. Pat. No. 5,937,809 and U.S. Pat. No. 6,019,076, the disclosures of which are incorporated herein by reference.
Generally, a conventional VVA mechanism includes a rocker arm that carries an input cam follower, such as a roller. The input cam follower engages an opening or input cam lobe of a rotating input shaft, such as the engine camshaft, and transfers rotation of the input cam lobe to oscillation of the rocker arm toward and away from the input shaft in a generally radial direction. The oscillation of the rocker arm is transferred via a link arm to pivotal oscillation of an output cam relative to the input shaft. The pivotal oscillation of the output cam is transferred to actuation of an associated valve by an output cam follower, such as, for example, a roller finger follower.
A desired valve lift profile is obtained by pivoting a control shaft into a predetermined angular orientation relative to a centerline thereof. A frame member of the VVA mechanism is pivotally disposed on the input shaft, and is coupled at one end thereof to the control shaft and at the other end thereof to the rocker arm. The pivotal movement of the control shaft is transferred, via the frame member, rocker arm and link arm, to pivotal movement of the output cam relative to a central axis of the input shaft. Thus, pivoting the control shaft places the output cam into the base or starting angular orientation. The base or starting angular orientation of the output cam, in turn, determines the portion of the lift profile thereof that will engage the output cam follower during pivotal oscillation of the output cam. The lift profile of the output cam that engages the cam follower determines the valve lift profile.
The rocker arm may carry a closing cam follower, such as, for example, a slider pad, that engages a closing cam lobe of the rotary input shaft. The closing cam lobe follows or lags the opening cam lobe. The closing cam follower transfers rotation of the closing cam lobe to the rocker arm, thereby ensuring that the output cam is pivoted back or returned to its starting or base angular orientation. Adding a closing cam to the camshaft of an engine, however, requires a redesigned camshaft and adds substantial complexity to the manufacture of, and thus adds cost to, the camshaft.
Alternatively, a biasing means, such as, for example, a spring, may be incorporated that biases the output cam back to the starting or base angular orientation. Such VVA mechanism are sometimes referred to as spring-based VVA mechanisms. The biasing means, typically referred to as a return or lash spring, engages, for example, the rocker or link arm of the VVA mechanism in such a way that the spring is compressed as the output cam is oscillated counter-clockwise from its starting position during actuation of the associated valve, and is expanded or decompressed during the closing of the associated valve. The expansion or decompression force of the spring pivots the output cam back to the starting or base angular position.
Springs, however, have a natural frequency or mode of vibration that is often referred to as spring surge. The operational frequency of a VVA mechanism that utilizes a return spring is limited to a maximum of approximately eight to ten times less than the natural frequency of the return spring. This limited maximum operational frequency of the VVA undesirably limits the maximum engine speed at which variable valve timing can be utilized. Further, the utilization of lash/return springs in VVA mechanisms limits the reliability of the mechanisms.
Therefore, what is needed in the art is a spring-based VVA mechanism that includes a lash or return spring and has an increased maximum operational frequency relative to other spring-based VVA mechanisms.
Furthermore, what is needed in the art is a spring-based VVA mechanism having increased reliability.
The present invention provides a spring-based variable valve actuation mechanism.
The present invention comprises, in one form thereof, output cams pivotally disposed upon an input shaft of an engine. First and second frame members are disposed upon the input shaft on respective sides of an input cam lobe of the input shaft. A link arm is pivotally coupled at a first end thereof to the output cams. A rocker arm assembly is pivotally coupled at a first end thereof to a second end of the link arm. The rocker arm assembly carries a cam follower that engages the input cam lobe of the input shaft. A biasing means is grounded to the first and second frame members, and biases the cam follower into engagement with the input cam lobe.
An advantage of the present invention is that the maximum operational speed of the VVA mechanism is increased relative to other spring-based VVA mechanisms.
A further advantage of the present invention is that the reliability of the VVA mechanism is improved over conventional spring-based mechanisms.